Either format the total hours worked in front end, or if this is not applicable something like:
SELECT CONVERT(varchar(10), CONVERT(int, SUM(HoursWorked))) + ':'
+ RIGHT('0' + CONVERT(varchar(2), CONVERT(int, FLOOR(60.0 * SUM(HoursWorked) + 0.5)) % 60), 2)
If the sum of the hours worked was guaranteed to be less than 24 hours then you could use the following, but I don't think this will work for you in this case.
SELECT CONVERT(varchar(5), DATEADD(minute, CONVERT(int, FLOOR(60.0 * SUM(HoursWorked) + 0.5)), 0), 108)